The invention relates to a process for preparing heat-vulcanizable silicone compositions. Particularly, the invention relates to a process for preparing heat-vulcanizable silicone compositions from an untreated filler and silicone polymer premix.
A heat-vulcanizable silicone composition comprises a high viscosity silicone polymer, an inorganic reinforcing filler and various additives that aid processing or impart desired final properties to the composition. A vulcanizing agent can be added and the composition heat-cured to fabricate silicone rubber moldings such as gaskets, medical tubing and computer keypads.
The heat-vulcanizable silicone composition can be prepared from a concentrate called a premix. A premix is an extrudable composition formed of a blend of a polymer and a finely divided filler, prepared by combining the materials under high shear force conditions in the presence of a liquid processing aid. The liquid processing aid treats filler silanol groups. Residual silanol groups on the surface of the filler can govern strength of hydrogen bonds between the silica and hydroxyl or oxygen groups in the silicone polymer chain. High concentrations of residual silanols in a filler cause “structuring” or “crepe hardening” of the final product in storage. This effect leads to difficulties in the processing of the material after it has been stored for extended periods. Typically, a treating agent is added to reduce silanol functional groups to a required concentration.
Large interfacial forces develop between silicone polymer and free, unreacted silanol groups present in filler. The treating agent must diffuse through a bulk of the high molecular weight silicone polymer and penetrate a rig˜ silicone polymer/filler interface to reach reactive silanol groups. Hence heretofore, the treating agent has been added either directly to the silica before the premix forming step or during the premix forming step under conditions of high shear mixing.
A treated premix is a damp composition that is more difficult to store or to transport than a dry mix. Hence, treated premix is prepared and immediately charged into a subsequent extrusion step. For example, Kasahara al., U.S. Pat. No. 5,198,171, discloses preparing a premix of polydiorganosiloxane, inorganic filler and liquid treating agents in a high speed mechanical shearing mixer. The resulting premix is then compounded in a same-direction double screw extruder.
The difficulties of storing and transporting a damp treated premix place limitations on premix use and marketability. There is a need for a filler and silicone polymer premix with extended use properties.